Organic and hybrid organic-inorganic nanoparticle solar cells are promising sources of low-cost, large area renewable energy due to their potential to be fabricated by simple coating and printing methods. In particular, CdSe/poly(3-hexylthiophene) (P3HT) based hybrid solar cells have demonstrated power conversion efficiency of about 2.8%. Despite their promise, hybrid organic/inorganic nanoparticles photovoltaic cells are not used in commercial devices. The primary barrier has been the propensity of the hybrid organic/inorganic nanoparticles solar cells to rapidly degrade in air. The mechanism of solar cell degradation is attributed to effects of oxygen and water in the active layers, as most semiconducting polymer materials degrade when exposed to humidity and/or oxygen. Additionally, photo-oxidation can be a serious problem with these materials.
The degradation of organic polymer based photovoltaic devices can be reduced to acceptable levels by sealing the components inside an impermeable package using glass and/or metal to prevent exposure to oxygen and water vapor. Attempts to create flexible packaging using hybrid multilayer barriers comprised of inorganic oxide layers separated by polymer layers with total thickness of 5-7 μm have displayed some promising results, but encapsulation methods that can reduce oxygen and moisture permeation, are expensive and typically result in increased device thickness and a loss of flexibility. To achieve flexibility and a sufficiently thin layer for printed plastic electronics, improved barrier materials are needed or a device with an inherently reduced sensitivity to moisture and oxygen is needed to enable large scale commercialization on plastic substrates.
For devices with a reduced sensitivity, Lee et al., Adv. Mater. 2007, 19, 2445 reports an ITO/PEDOT:PSS/Active-Layer/TiOx Amorphous layer/Al device, as shown in FIG. 1. The titanium oxide layer between the low work function aluminum cathode and active layer of a polymer solar cell can significantly improved the device durability. The ability of titania (TiO2) to have substantial oxygen/water protecting and scavenging effects was established, and the effect originates from TiO2's ability to act as a photocatalysis and to an inherent oxygen deficiency of TiO2. Typically, crystalline TiO2 layers are prepared at temperatures above 450° C. which is inconsistent with a process for the fabrication of polymer electronic devices. Lee et al. developed a solution-based sol-gel process that allows fabrication of a titanium sub-oxide (TiOx) layer on a polymer-based active layer as the collector and optical spacer for a polymeric solar cell. Unfortunately, the power conversion efficiency of the solar cell with a titanium sub-oxide (TiOx) layer deceased by about 50% after 6 days in a glove box ambient.
Hau et al., Appl. Phys. Lett. 2008 92, 253301 reports an ITO/ZnO NPs/Active-Layer/PEDOT:PSS/Ag inverted device, as shown in FIG. 2, where ZnO NPs are ZnO nanoparticles. The ZnO NPs were used because of their good electron mobility without a thermal post-treatment. The ZnO nanoparticle layer on ITO/glass as well as ZnO NPs on ITO-coated plastic substrates were comparable in stability to those of conventional devices using LiF/Al as an electrode on glass substrate. An improved stability for the device is attributed to the PEDOT:PSS layer and Ag electrode, where the PEDOT:PSS layer acts as a barrier that prevents oxygen from entering the active layer and the ability of the Ag electrode to form a layer of silver oxide in air that increases its effective work function. The solar cells exhibited only a 20% loss of power conversion efficiency after 40 days in air. Unfortunately the cost of silver, or other less air sensitive high work function metals, such as gold, is prohibitively expensive to be substituted for Al electrodes.
Hence, there remains a need for a hybrid organic-inorganic nanoparticle solar cell that displays a good efficiency and stability at a viable price to allow commercialization.